Most modern railroad locomotives are of the diesel-electric variety, in which a diesel engine drives electrical generating apparatus to power one or more electric motors to turn the locomotive wheels. The engine is typically turbocharged and includes an aftercooler to remove the heat of compression from the turbocharged air before it enters the engine. The engine cooling system, which circulates a liquid coolant through an engine coolant loop to remove heat from the engine, is supplemented by an aftercooler coolant loop to remove heat from the aftercooler.
U.S. Pat. No. 5,598,705, Turbocharged Engine Cooling Apparatus, issued Feb. 4, 1997, describes cooling apparatus for a diesel electric locomotive which employs separate engine and aftercooler coolant loops, each with its own radiator and pump apparatus and separate coolant conduits but sharing a single coolant tank. The loops are also connected by a linking conduit connecting the outlet of the aftercooler radiator with the inlet of the engine coolant pump. A valve in the linking conduit can be closed to prevent linking coolant flow; and the system is designed to operate in that manner under normal conditions, with the engine radiator and conduits sized to provide sufficient cooling for the engine and optional oil cooler in normal, warmed up operation while the aftercooler radiator provides cooling of the turbocharged air for maximum fuel economy and low emissions. The engine radiator is not designed to provide sufficient engine cooling for extremely hot running conditions, but the valve can be opened as necessary in such conditions to admit low temperature coolant from the aftercooler coolant loop to the engine cooling loop for extra engine cooling, with return flow through the coolant tank.
In such a locomotive engine, however, the engine provides a much greater heat load than the aftercooler; and such a cooling system designed for operation under normal conditions with a closed linking valve thus requires significantly greater cooling capacity in the engine radiator than in the aftercooler radiator. Due to size and shape constraints in the crowded locomotive body, it is customary to provide the radiators in two long, narrow banks of equal size, the banks being aligned side by side and extending fore/aft with respect to the locomotive, as seen in FIG. 2 of this document. Several fans are positioned above the radiator banks to maintain and control cooling air flow through the radiators; and efficient design dictates that these fans are arranged in a fore/aft alignment over the radiator banks and that each affects air flow through both radiator banks simultaneously. The flow tubes of each radiator bank are divided between the engine and aftercooler coolant loops in a ratio (e.g. 3:1) determined to provide a greater cooling capacity in the engine coolant loop than in the aftercooler coolant loop, so as to balance the different heat loads in normal, warmed up operation. This requires a conduit arrangement to provide coolant from each loop through portions of each radiator bank; and this arrangement is complex and expensive due to the extra pipes and expansion joints required.
The conduction of coolant from each loop to each radiator bank also produces thermal stresses in the radiator banks which must be limited. The flow tubes of a radiator bank are attached at each end to plates; and thermally induced elongations of the tubes will differ between those tubes conducting hotter coolant from the engine coolant loop and cooler coolant from the aftercooler coolant loop. Such different elongations produce rotational torques on the end plates which stress the tube/plate joints and can thus cause radiator damage if the temperature difference between the engine and aftercooler coolant loops becomes too great. The need to limit this temperature difference is a control constraint that places an otherwise unnecessary limit on the use of the aftercooler to increase fuel economy and decrease undesirable emissions at cooler temperatures and thus prevents optimal operation of the engine.
This cost and complexity in flow conduits and control limitations could be reduced or eliminated if one of the radiator banks were dedicated totally to the engine coolant loop and the other were dedicated totally to the aftercooler coolant loop. But this would unbalance the system, since the engine would provide a heat load to the engine radiator bank up to three times greater than the aftercooler provided to the aftercooler radiator bank of similar size and cooling capacity. In addition, the flow tubes of the radiator banks have a maximum flow rate, beyond which they are subject to cavitation damage. Therefore, assuming the system has been designed for coolant flow rates making efficient use of the radiator banks, the larger heat load provided to the engine radiator bank can not be absorbed by further increasing the coolant flow rate through the bank. Thus, the system as described would have insufficient cooling capacity in the engine cooling loop and unused excess cooling capacity in the aftercooler cooling loop to operate with the linking valve closed in normal, warmed up operation. Although the linking valve could be kept open in normal operation to provide linking coolant flow for additional engine cooling, this is not desirable. The linking valve used in the aforementioned system is typically a ball type valve; and extended operation in a single, partly opened position tends to shorten the life span of such a valve. Furthermore, such operation would reduce the operational control range of the valve and allow the possibility of engine overheating if the valve is accidentally closed completely. Moreover, the system as a whole is not optimally designed for such operation. All coolant exiting the engine passes through and is cooled by the engine radiator before being mixed with coolant from the aftercooler; and the hottest coolant in the system, which is found at the outlet of the engine, is thus not directly involved in the mixing. This can produce hotter engine temperatures for a given linking valve opening and slower response to increasing engine temperature.